Duncan Steel, a Visiting Researcher at the Australian Centre for Astrobiology at the
University of New South Wales, responded.

There is no doubt that there was an impact on Earth by a large (10 to 15 km) asteroid
or comet at very close to the time of the well-known dinosaur extinction 65 million
years ago. The scar in the Earth's crust - a heavily-eroded crater over 200 km in
diameter - has been identified on the Yucatan Peninsula in Mexico. This phenomenal
explosion distributed dust and rock around the globe. The initial evidence for the
impact, identified before the crater was mapped, came from an anomalously large
amount of the element iridium being found in the geological strata laid down at the
time of the disappearance of the dinosaurs, the iridium being detected first in rocks
in Italy, Denmark, and New Zealand. Iridium is common in meteorites, but not
in Earth’s crustal rocks.

Over the past decade or so other terrestrial craters of about the right age
(65 million years) have been found and dated, implying that there may have been
several impacts. This may have been due to an asteroid or comet breaking apart
before various large fragments struck our planet.

The situation is confused, however, by the fact that a huge set of volcanic eruptions
also occurred at about that time. These have left a type of rock called flood basalt
over an area of thousands of square kilometres of India. These are known as the
Deccan Traps.

The volcanism and the impacts may have been simply coincidental, producing a
double-whammy that killed the dinosaurs, or else it has been suggested that a
really large extraterrestrial object perhaps 50 km across may have struck India
and smacked a hole into the crust deep enough such that volcanic flows followed.
Geologists speak of that impact being 'self-erasing' in that the eruptions, which
continued for millennia, wiped out the local evidence for their origination.

What killed the dinosaurs? Many of them were grilled to death as the sky lit up
from the meteors produced by the atmospheric re-entry of rocks thrown out by the
impact(s). Those that survived froze to death as dust blanketed the atmosphere for
some years. Then there was the highly-acid rain resulting from the vapourised sulphate
and carbonate rocks. Next the global temperature was elevated for millennia because
of the carbon dioxide from those carbonates. And lots of other nasty knock-on effects:
a calamity like a big impact really upsets the terrestrial environment for an extended
period of time, causing geological boundaries that are obvious in the rock strata.